The typical asking rate per listing is in the $20's per square foot per year. The O'Fallon submarket is experiencing a an increase in rates. In the last 3 months, the median rate went up $2 per sqft per year. That's a 10% rise in rent.
In the last 90 days, 31 listings have been posted online, which means new availabilities are probably leasing quickly. You'll want to be prepared to tour spaces as quick as possible. About 3% of the available leases in the O'Fallon are subleases, so you will probably need to consider the implications of signing a direct lease with the building. There aren't many listings below 2,000 square feet on the market, so finding a space for a small business might be tough.
The standard measurement for office space in this area is around 1,360 sqft. Most buildings in this market are around 2 stories. Most buildings in this region were built in 2001 and are Class A.